FIG. 2(a) shows a cross-sectional view of a prior art buried type PBT. In FIG. 2(a), reference numeral 3 designates a source (emitter) n.sup.+ semiconductor layer. An n.sup.+ substrate or an epitaxially grown layer is used for this source layer 3. A channel layer 1 is epitaxially grown on the source n.sup.+ semiconductor layer 3. A drain (collector) n.sup.+ semiconductor layer 4 is disposed on the channel layer 1. Gates (bases) 2 comprising a grating of a Schottky barrier metal are arranged centrally in the channel layer 1. Gate depletion layers are broadened in a region at the neighborhood of gates 2 during transistor operation.
FIGS. 2(b) and 2(c) show cross-sectional views of a prior art dug side wall type PBT and a prior art dug edge type PBT, respectively. In the digged side wall type PBT in FIG. 2(b), grooves reaching the central portion of channel layer 1 from the surface of drain layer 4 are produced and gates 2 are disposed in the grooves. In the dug edge type PBT shown in FIG. 2(c), gates 2 are disposed in the grooves and the gates 2 are processed to produce a trapezoidal cross-sectional configuration.
Although the cross-sectional structures of above-described PBTs are different from each other, in all PBTs, the base region including a grating of thin-film Schottky metal gates and a current permeable channel portion, and the operating current (channel current) flows in the vertical direction, that is, transverse to the substrate thickness.
The main operation thereof is as follows. When an input control signal is applied to the Schottky metal gate as a base, the gate depletion layer is modulated, whereby the gate current at the channel is modulated.
These PBTs have operational characteristics as in the following.
1) Since the base region is a longitudinal type structure in which the channel current flows in the substrate thickness direction, the thickness of the gate metal corresponds to the gate length, whereby quite short gate lengths of about 0.1 micron can be easily realized. A super high frequency operation is expected in the device of such a longitudinal type structure.
2) Since the active layer between the drain and source layer is produced by an epitaxial growth method, the film thickness is as thin as about 0.2 to 0.5 microns. In a compound semiconductor including charge carriers having a small effective mass, such as GaAs, ballistic electronic conduction arises in such thin active layer, thereby reducing gate propagation delay. This results in super high speed operation.
3) Since the input control signal is applied to the gate depletion layer capacitance through the gate metal, the loss due to parasitic resistances is less than with bipolar transistors in which the input control signal is applied through the base.
4) In a structure where the n.sup.+ substrate is used for the source n.sup.+ layer, grounding with a quite small amount of inductance can be realized, thereby resulting in a transistor which is suitable for a high frequency and high power operation.
In the prior art longitudinal type PBT structure, however, since the gate electrode 2 is positioned at the central portion of the channel layer 1 as an epitaxial growth layer as shown in FIG. 2(a), the quality of semiconductor crystal is poorer at the upper half portion of channel layer 1, resulting in difficulty in increasing the breakdown voltage. Furthermore, in the PBT structures shown in FIGS. 2(b) and 2(c), although the deterioration of crystallinity is not a great problem because no semiconductor layer is disposed on the gate electrode 2, separation of the gate electrode 2 from the drain electrode 4 is structurally difficult.